1. Field of the Invention
This invention relates in general to a device for identifying an individual by identification of his/her fingerprint. In particular, this invention relates to a fingerprint sensor for transforming the information contained in a person's fingerprint into an electric output signal. Still more particularly, this invention relates to a fingerprint sensor using optical means for reading a fingerprint.
2. Description of the Prior Art
Fingerprint identification systems which identify the print of a finger pressed on a contact surface are well-known in the art.
U.S. Pat. No. 4,053,228, for instance, discloses a finger identification apparatus which contains a transparent glass plate serving as a contact surface or fingerprint reader. A fingerprint is formed by pressing the finger under investigation against the back surface of the glass plate and holding it in a predetermined position thereon. The fingerprint is interrogated by a light beam directed through the front surface of the glass plate. The interrogating beam is partially reflected at the back surface to provide a signal beam which carries the fingerprint information. The reflected signal beam is then correlated against a hologram of the same fingerprint in order to provide the identification of the individual.
In U.S. Pat. No. 4,120,585, another fingerprint identification system is disclosed. This system contains a pliable optical prism as a fingerprint sensor. The base of the prism is physically contacted by the finger of the person under investigation. The pliable prism deforms under the applied pressure. It partially reflects a sensing light beam to a photo-sensitive device which will be activated. The photo-sensitive device, in turn, activates further optical components of the fingerprint identification system. A fingerprint reader is examined for the ridge-valley pattern of the fingerprint of a person to be identified. It should be mentioned that in this prior art system the prism surface is deformed or bent as a whole. The finger does not impress a pattern into the surface in order to obtain therein the configuration of the topographical relief of the finger.
The prior fingerprint identification systems require highly sophisticated technology. Yet, a fingerprint identification system should easily and inexpensively be assembled, and simultaneously it should afford a high image quality of the fingerprints. Particularly, the fingerprint image should be free from distortions.